Ion conductor having a garnet structure

ABSTRACT

The present invention relates to the use of chemically stable solid ion conductors having a garnet-like structure in batteries, accumulators, electrochromic devices and other electrochemical cells, and also novel compounds which are suitable for these uses.

The present invention relates to the use of chemically stable solid ionconductors having a garnet-like structure in batteries, supercapacitors,accumulators and electrochromic devices, chemical sensors andthermoelectric converters, and also novel compounds which are suitablefor these uses.

Rechargeable (secondary) batteries are used where grid-independentoperation of electric and electronic appliances is necessary or desiredfor at least part of the time. Research on solid ion conductors aselectrolyte materials for this use forms, in this context, an importantaspect of current material research. The advantages sought in a batterycomposed only of solids are guaranteed freedom from leaks,miniaturizability, electrochemical stability, relatively high energydensities and a relatively long life.

Among the various battery technologies, battery systems based on lithiumions have become increasingly established in recent years. They areparticularly notable for their high achievable electric energy densityand power, which are attributable to the high chemical reactivity andthe low mass of lithium ions and also their high mobility. Thedevelopment of solid lithium ion conductors has attracted considerableattention in recent years. Examples are Li_(2.9)PO_(3.3)N_(0.46) or Li₃Nand Li-β-aluminium oxide. However, Li_(2.9)PO_(3.3)N_(0.46) has asignificantly lower ion conductivity than liquid electrolytes. Li₃N andLi-β-aluminium oxide are very sensitive to moisture. In addition, Li₃Ndecomposes at a voltage as low as 0.445 V at room temperature andLi-β-aluminium oxide is not chemically stable.

Lithium ion conductors having a garnet-like structure were described forthe first time in the study by Thangadurai et al., “Novel Fast LithiumIon Conduction in Garnet-Type Li₅La₃M₂O₁₂ (M=Nb, Ta)”, J. Am. Ceram.Soc. 86, 437-440, 2003. The garnet-like Li₅La₃M₂O₁₂ compounds have anappreciable lithium ion conductivity.

In structural terms, garnets are orthosilicates of the generalcomposition X₃Y₂(SiO₄)₃ which crystallize in the cubic crystal system,where X and Y are octacoordinated and hexacoordinated cation sites. Theindividual SiO₄ tetrahedra are connected to one another by ionic bondsvia the interstitial B cations.

The garnet-like compounds of the formula Li₅La₃M₂O₁₂ (M=Nb, Ta) whichare described in the above-mentioned study by Thangadurai et al. containan excess of Li ions compared to an ideal garnet structure. The La³⁺ andM⁵⁺ ions occupy the octacoordinated and hexacoordinated sites, whilelithium ions occupy positions having six-fold coordination.

The PCT application WO 2005/085138 reports that further garnet-likelithium ion conductors are obtained formally by aliovalent substitutionfrom the compounds of the formula Li₅La₃M₂O₁₂ (where M=Nb or Ta).Aliovalent substitution of the La³⁺ sites can increase the connectivityof the network and enables the number of available vacancies to bevaried. Charge balance is preferably achieved by means of Li⁺ ions (L).For the purposes of the present invention, “aliovalent substitution”means the replacement of an ion by an ion having a different oxidationstate, as a result of which cation vacancies, anion vacancies,interstitial cations and/or interstitial anions are formed. The solidlithium ion conductors are chemically stable and have an ionconductivity of more than 3.4×10⁻⁵ S/cm. Owing to their high ionconductivity accompanied by negligible electron conductivity, they canbe used as solid-state electrolytes.

The compounds described in WO 2005/085138 generally have thestoichiometric composition L_(5+x)A_(y)G_(z)M₂O₁₂, where

L is in each case independently any preferred monovalent cation,A is in each case independently a monovalent, divalent, trivalent ortetravalent cation,G is in each case independently a monovalent, divalent, trivalent ortetravalent cation,M is in each case independently a trivalent, tetravalent or pentavalentcation,0≦x≦3, 0≦y≦3, 0≦z≦3 andO can be partly or completely replaced by divalent and/or trivalentanions such as N³⁻.

In the ion conductors described M is in each case one of the metals Nband Ta. Other examples of metal ions are not given. Ion conductionoccurs via lithium ions (L=Li).

Further examples of lithium ion conductors having a garnet structurehave been examined in recent years (V. Thangadurai, W. Weppner, Adv.Funct. Mater. 2005, 15, 107-112; V. Thangadurai, W. Weppner, J. PowerSources, 2005, 142, 339-344). Here, Li₆BaLa₂Ta₂O₁₂ had the highest Li⁺ion conductivity of 4×10⁻⁵ Scm⁻¹ at 22° C. with an activation energy of0.40 eV. While Li₆BaLa₂Ta₂O₁₂ is stable towards reaction with metalliclithium, moisture, air and customary electrode materials, the volumeconductivity and total conductivity at room temperature are still notsufficiently high to enable an ideal rechargeable solid lithium ionbattery to be developed.

Another problem associated with the above ion conductors of the priorart is that the proposed metals niobium and tantalum are relativelyexpensive and not readily available. In addition, the use of a solidelectrolyte which consists entirely of the garnet-like compoundsdescribed is complicated and associated with high costs.

It was therefore an object of the present invention to provide improvedsolid ion conductors in which the above disadvantages are at leastpartly overcome.

It has now been found, according to the invention, that zirconium can beused as metal M in the garnet-like ion conductors. In contrast toniobium and tantalum, zirconium is readily available and leads to verystable solid-state structures. While Nb and Ta are formally present inthe oxidation state +V in the garnet structure, Zr is preferably in theoxidation state +IV.

The invention therefore provides, in one embodiment, a solid ionconductor which has a garnet-like crystal structure and has thestoichiometric composition L_(7+x)A_(x)G_(3−x)Zr₂O₁₂, where

-   -   L is in each case independently a monovalent cation,    -   A is in each case independently a divalent cation,    -   G is in each case independently a trivalent cation,    -   O≦x≦3 and    -   O can be partly or completely replaced by divalent or trivalent        anions such as N³⁻.

L is particularly preferably an alkali metal ion, for example Li⁺, Na⁺or K⁺. In particular, combinations of various alkali metal ions are alsopossible for L. In a particularly preferred embodiment of the invention,L=Na⁺. Sodium is very inexpensive and available in any amounts. Thesmall Na⁺ ion can move readily in the garnet-like structures and incombination with zirconium gives chemically stable crystal structures.

A is any divalent cation or any combination of such cations. Divalentmetal cations can preferably be used for A. Particular preference isgiven to alkaline earth metal ions such as Ca, Sr, Ba and/or Mg and alsodivalent transition metal cations such as Zn. It has been found thatthese ions move very little if at all in the garnet-like compoundsaccording to the invention, so that ion conduction occurs essentiallyvia L.

In the above composition, preference is also given to 0≦x≦2 andparticularly preferably 0≦x≦≦1. In an embodiment according to theinvention, x=0, so that A is not present in the garnet-like compound.

G is any trivalent cation or any combination of such cations. Trivalentmetal cations can preferably be used for G. Particular preference isgiven to G=La.

In a structure of the above composition, O²⁻ can be partly or completelyreplaced by other anions. For example, it is advantageous to replace O²⁻completely or partly by other divalent anions. Furthermore, O²⁻ can alsobe aliovalently replaced by trivalent anions with appropriate chargecompensation.

In a further aspect; the present invention provides a solid ionconductor of the stoichiometric composition. L_(7+x)A_(x)La_(3−x)Zr₂O₁₂,where A is a divalent metal and L is Li or Na. Because of its readyavailability, Na is particularly preferred. In a preferred embodiment,x=0, so that the composition is L₇La₃Zr₂O₁₂.

A is preferably selected from among alkaline earth metals, preferablyfrom among Ca, Sr, Ba and/or Mg. Preference is likewise given to A beingselected from among divalent transition metals, for example A=Zn.Greatest preference is given to A=Sr or Ba.

Ion conductors of the composition L_(7+x)A_(x)La_(3−x)Zr₂O₁₂ have agarnet-like crystal structure. Compared to the known compounds of thecomposition L₅La₃Nb₂O₁₂ (L=Li), the two Nb(+V) cations have formallybeen replaced by two Zr(+IV) cations and two monovalent L cations. Inaddition, La(+III) may have been replaced by A(+II) and L(+I). In thisway, the total proportion of L in the structure has been increased. L ispreferably Li or Na, via which the ion conduction of the compoundshaving a garnet structure occurs. As a result, the compounds of thepresent invention make it possible to provide significantly improved ionconductors.

Compared to the compounds of the prior art, the materials of thecomposition L_(7+x)A_(x)La_(3−x)Zr₂O₁₂ display an increased ionconductivity. Owing to the garnet structure of the compounds of thepresent invention, which is a 3D-isotropic structure, ion conduction inthree dimensions without a preferential direction is possible.

The electronic conductivity of the compounds of the present inventionis, on the other hand, comparatively low. The polycrystalline samples ofthe compounds of the present invention also have a low grain boundaryresistance, so that the total conductivity is made up virtuallyexclusively of the volume conductivity.

A further advantage of the materials is their high chemical stability.The materials display, in particular, no discernible changes on heatingin contact with molten lithium. At temperatures up to 350° C. and DCvoltages up to 6 V, no chemical decomposition is observed.

An example of a particularly preferred compound according to theinvention having a garnet structure is Li₇La₃Zr₂O₁₂. The high lithiumion conductivity, good thermal and chemical stability in respect ofreactions with possible electrodes, environmental compatibility,availability of the starting materials, low manufacturing costs andsimple production and sealing make Li₇La₃Zr₂O₁₂ a promising solidelectrolyte which is particularly suitable for rechargeable lithium ionbatteries.

According to a further aspect, the present invention provides a processfor preparing the solid ion conductors having a garnet-like structure.The compounds can be formed by reaction of appropriate salts and/oroxides of the elements present, for example by means of a solid-statereaction. Particularly useful starting materials are nitrates,carbonates and hydroxides which are converted into the correspondingoxides during the course of the reaction.

The present invention more specifically relates to a process forpreparing the solid ion conductors of the compositionL_(7+x)A_(x)G_(3−x)Zr₂O₁₂ (e.g. Na₆ALa₂Zr₂O₂₂). The materials can beobtained by reaction of appropriate salts and/or oxides of A, G and Zrwith a hydroxide, nitrate or carbonate of L in a solid-state reaction. Ais as defined above. The divalent metal A is preferably used in the formof nitrates. Here, preference is given to Ca(NO₃)₂, Sr(NO₃)₂ andBa(NO₃)₂. In the case of G, preference is given to using La which ispreferably employed in the form of La₂O₃. Zr is advantageously used asoxide, preferably ZrO₂. L is preferably used in the form of LOH, LNO₃ orL₂CO₃. For example, LiOH.H₂O or NaOH.H₂O can preferably be used. Tocompensate for a weight loss of L (e.g. L=Li, Na) during the heattreatment of the samples, the respective salt is preferably used inexcess, for example an excess of 10% by weight.

The starting materials are mixed in a first step and can, for example,be milled in 2-propanol in a ball mill using zirconium oxide millingmedia. The mixture obtained in this way is subsequently heated attemperatures in the range of preferably 400-1000° C. in air for a numberof hours, preferably 2-10 hours. Temperatures of 600-800° C., forexample about 700° C., and a heat treatment time of 4-8 hours e.g. about6 hours, are particularly suitable. Milling is then carried out again,preferably likewise in 2-propanol in a ball mill using zirconium oxidemilling media. The reaction product is subsequently pressed uniaxiallyor preferably isostatically to give moulded pieces, for example pellets.These are then sintered for a number of hours, preferably 10-50 hours,more preferably 20-30 hours, at temperatures in the range of preferably700-1200° C., more preferably 800-1000° C. Temperatures of about 900° C.and a heat treatment time of about 24 hours are particularly suitablehere. During this sintering process, it is advantageous to cover thesamples with a powder of the same composition in order to avoidexcessive losses of the L oxide.

Possible methods which can easily be employed for preparing the compoundare precursor methods, e.g. the Pecchini method, the glycine method orprecipitation reactions, since soluble salts exist for all components.

The solid ion conductors of the invention (e.g. lithium or sodium ionconductors) are, as solid-state electrolytes, a valuable startingmaterial. Since the materials have an extraordinarily high ionconductivity accompanied by negligible electron conduction, they can beused as solid electrolyte for batteries (e.g. lithium or sodiumbatteries) having a very high energy density. The high stability of thematerials in respect of chemical reactions, e.g. with elemental lithiumand customary electrode materials, leads to, for example, the solid ionconductors of the present invention being able to be put to practicaluse in batteries.

The resistance of the phase boundary between the solid electrolytes ofthe present invention and the electrodes is also very small compared tocustomary solid electrolyte materials. As a result, batteries having acomparatively high power (high currents) can be produced using thematerials according to the invention. The use of the solid-stateelectrolytes of the present invention also results in improved safetycompared to the use of liquid electrolytes. This is of particularadvantage when the electrolytes are used in motor vehicles.

In a further aspect, the present invention provides for, apart from theuse in batteries, the use of the solid ion conductors (e.g. lithium ionconductors) in electrochromic systems (windows, VDUs, exterior walls,etc.) and for instantaneous energy storage and release insupercapacitors (supercaps). When the ion conductors of the inventionare used, it is possible to achieve energy densities of capacitors of100 F/cm³ or more. A further aspect of the invention is the use of thegarnet-like solid ion conductors as sensors, in particular for numerousgases. According to the invention, it is also possible to use thematerial in thermoelectric converters for efficient direct conversion ofheat into electric energy.

The ion conductors having garnet-like structures can also be used asbuffer layers in combination with other electrolytes, for exampleconventional aprotic liquid electrolytes. It is therefore not necessaryto use an electrolyte which consists entirely of the garnet-likestructure. Rather, it is possible to use any known electrolytes whichcan, for example, be present in liquid, gel or solid form in combinationwith the novel garnet-like ion conductors.

The invention therefore provides, in a further aspect, for the use of asolid ion conductor having a garnet-like crystal structure as protectivelayer before an electrode so as to improve the chemical stabilitytowards the electrolyte. For this purpose, it is possible to use notonly the garnet-like structures according to the invention containingzirconium but also, for example, the garnet-like compounds described inWO 2005/085138. The use of the ion conductors as buffer structure beforethe electrodes prevents short circuits and makes it possible to generateand apply relatively high voltages so as to achieve a significantlygreater energy density and life.

FIGURES

FIG. 1:

AC impedance curve of Li₁La₃Zr₂O₁₂, measured at 18° C. in air on a thickpellet (1.02 cm thick and 0.92 cm in diameter). The continuous linerepresents the simulated data for an equivalent current circuitcomprising (R_(b)Q_(b)) (R_(gb)Q_(gb)) (Q_(el)) (where R is theresistance and Q is the constant phase element and the indices g, gb andel indicate grain volume, grain boundary and electrode) using theEQUIVALENT Program (B. A. Boukamp, Equivalent Circuit, Version 4.55,1997, Faculty of Chemical Technology, University of Twente, 7500 AEEnschede (The Netherlands), Report No. CT88/265/128/CT89/214/128; May1989). The impedance curve measured at 18° C. in air on a thin pellet(0.18 cm thick and 0.98 cm in diameter) of Li₇La₃Zr₂O₁₂ is shown in theinset.

FIG. 2:

a) Arrhenius curves for the electrical volume and total conductivity(volume and grain boundaries) of the thick pellets of Li₇La₃Zr₂O₁₂,obtained in two successive heating and cooling cycles.

b) Comparison of the Arrhenius curves obtained for the thick and thinpellets of Li₇La₃Zr₂O₁₂ during the first heating run (18-300° C.).

FIG. 3:

Comparison of the total conductivity (volume+grain boundaries) ofLi₇La₃Zr₂O₁₂ and other known lithium ion conductors which come intoquestion for battery applications.

FIG. 4:

Measured powder XRD pattern of Li₇La₃Zr₂O₁₂ and standard pattern of theknown garnet phase Li₅La₃Nb₂O₁₂ (JCPDS: 80-0457) as per Joint Committeeon Powder Diffraction Standards.

FIG. 5:

AC impedance curves measured at 25 and 50° C. in air on the thick pelletof Li₇La₃Zr₂O₁₂.

FIG. 6:

AC impedance curves measured at 25 and 50° C. in air on the thin pelletof Li₇La₃Zr₂O₁₂. The further curve at higher frequency is shown as aninset.

FIG. 7:

Arrhenius curves for the electrical volume and total (volume+grainboundary) conductivity of the thin pellet of Li₇La₃ZrO₁₂, obtained intwo successive heating and cooling cycles.

FIG. 8:

Photographs of a) Li₇La₃Zr₂O₁₂ pellet and molybdenum crucible beforeexposure to molten lithium, b) Li₇La₃Zr₂O₁₂ pellet in molten lithium andc) Li₇La₃Zr₂O₁₂ pellet and molybdenum crucible immediately afterexposure to molten lithium for 48 hours. The photograph depicted inFigure c) shows that the colour of the pellet remains unchanged (ivorycolour) and no reaction product is formed.

The following example serves to illustrate a particularly preferredembodiment of the present invention.

EXAMPLE

Stoichiometric amounts of the in each case highly pure startingmaterials:

LiOH (Alfa Aesar, >99%), predried at 200° C. for 6 h, 10% by weightexcess in order to compensate for the Li loss during the sinteringprocess;La₂O₃ (Alfa Aesar, >99.99%), predried at 900° C. for 24 h; and

ZrO₂ (Aldrich, >99%)

were reacted in a solid-state reaction.

The starting materials were ball-milled for about 12 hours in 2-propanolusing zirconium oxide containers and balls. This was followed by heattreatment at 900 and 1125° C. in air for 12 hours. The product obtainedwas then once again ball-milled. The reaction products were subsequentlyisostatically pressed to form pellets and sintered at 1230° C. for 36 h.The samples were covered with a powder having the same compositionduring this procedure in order to avoid an excessive loss of lithium.The heating rate in all treatments was 1° C. per minute. The sinteredcompressed pellets were cut into thinner pellets by means of a diamondsaw. Phase formation was monitored using X-ray powder diffraction (XRD)(SEIFERT 3000, CuK_(α), Germany). The lattice constants were determinedfrom the powder XRD data using the method of least squares.

The measurement of the electrical conductivity was carried out in airusing two pellets of differing thickness (thick pellet: 1.02 cm thickand 0.92 cm in diameter, and thin pellet: 0.18 cm thick and 0.98 cm indiameter). The measurement was carried out using Li-ion-blocking Auelectrodes (Au paste cured at 700° C. for 1 h) in the temperature rangefrom 18 to 350° C. by means of an impedance and gain phase analyzer (HP4192 A, Hewlett-Packard Co., Palo Alto, Calif.) (5 Hz-13 MHz). Beforeeach impedance measurement, the samples were equilibrated at constanttemperature for from 3 to 6 hours. The impedance measurements werecarried out in two successive heating and cooling cycles for eachpellet. The data for the thermogravimetric analysis (TGA) anddifferential thermal analysis (NETZSCH STA 409 C/CD) were measured inair over the temperature range 29-900-20° C. at a heating and coolingrate of 2° C. per minute and isothermally at 900° C.

The stability of Li₇La₃Zr₂O₁₂ towards molten lithium was examined in anargon-filled glove box by reacting the pellet with a large excess ofmolten lithium in a molybdenum crucible for 48 hours.

Although numerous X-ray diffraction (XRD) studies have been carried outon Li₅La₃M₂O₁₂ (M=Nb, Ta) garnets, there has been controversy about thestructure in respect of the space group and position of the lithiumcations (a) D. Mazza, Mater. Lett. 1988, 7, 205-207; b) H. Hyooma, K.Hayashi, Mater. Res. Bull. 1988, 23, 1399-1407; C) J. Isasi, M. L.Veiga, R. Saez-Puche, A. Jereze, C. Pico, J. Alloys Compd. 1991, 177,251-257). Recently, neutron diffraction studies have indicated thatLi₅La₃M₂O₁₂ (M=Nb, Ta) crystallizes in the space group Ia3d and that Liis located both in the tetrahedral positions and octahedral positionsand that vacancies are present in both types of positions (a) E. J.Cussen, Chem. Commun. 2006, 412-413; b) M. P. O'Callaghan, D. R. Lynham,E. J. Cussen, G. Z. Chen, Chem. Mater. 2006, 18, 4681-4689). Themeasured powder XRD pattern of Li₇La₃Zr₂O₁₂ agrees well with thestandard pattern of the known garnet phase Li₅La₃Mb₂O₁₂ and demonstratesthe ability of the garnet structure to incorporate cations of differingoxidation state and different size without an excessive change in thesymmetry. The diffraction pattern for a cubic cell having a latticeconstant of A=12.9682 (6) Å was determined.

A typical impedance curve obtained at 18° C. for a thick pellet ofLi₇La₃Zr₂O₁₂ is shown in FIG. 1. The occurrence of the rise in theregion of low frequencies when the electrodes are ionically blocked isan indication that the material examined is an ion conductor (a) V.Thangadurai, R. A. Huggins, W. Weppner, J. Power Sources 2002, 108,64-69; b) J. T. S. Irvine, D. C. Sinclair, A. R. West, Adv. Mater. 1990,2, 132-138). Similar behaviour has been observed for the previouslystudied materials having a garnet-like structure. The impedance curvecould be resolved into volume, grain boundary and electrode resistances.The continuous line in FIG. 1 represents the data for an equivalentcurrent circuit of (R_(b)Q_(b)) (R_(gb)Q_(gb)) (Q_(el)) using theEQUIVALENT Program. The impedance curve for the thin pellet ofLi₇La₃Zr₂O₁₂ measured at 18° C. is shown as an inset in FIG. 1. Thevolume and total conductivity of the thick pellet (1.02 cm thick and0.92 cm in diameter) and the thin pellet (0.18 cm thick and 0.98 cm indiameter) of Li₇La₃Zr₂O₁₂ observed at various temperatures were obtainedfrom the intersections of the high-frequency and, low-frequencysemicircles with the axis and are summarized in Table 1. The data shownin FIG. 1 and Table 1 indicate similar electrical properties for thethick and thin pellets of Li₇La₃Zr₂O₁₂. The thin pellet displays aslightly higher volume and total conductivity compared to the thickpellet. In addition, it is interesting to note that the grain boundarycontribution to the total resistance is less than 50% and decreases withincreasing temperature (Table 1) both for the thick pellet and for thethin pellet. At higher temperature (above 75° C. for the thick pelletand above 50° C. for the thin pellet), it is difficult to determine thegrain boundary contribution compared to the volume contributionprecisely; the total value of the volume and grain boundarycontributions has therefore been shown for the determination of theelectrical conductivity over the temperature range examined. The totalconductivity at room temperature (3×10⁻⁴ S/cm at 25° C.) of the novelcrystalline fast lithium ion conductor Li₇La₃Zr₂O₁₂ having a garnet-likestructure is better than that of all other solid lithium ion conductorsand all previously described lithium garnets.

This result, viz. that total and volume conductivities are of the sameorder of magnitude, is a particularly advantageous property of theLi₇La₃Zr₂O₁₂ garnet structure examined here compared to other ceramiclithium ion conductors. For many applications of solid electrolytes inelectrochemical devices such as batteries, sensors and electrochromicdisplays, the total conductivity should be as high as possible. Inaddition, volume and total conductivity can be improved further by meansof a low-temperature synthesis of Li₇La₃Zr₂O₁₂ and by means of furtherdensification using a suitable sintering process.

The Arrhenius curves for the electrical volume and total conductivity ofLi₇La₃Zr₂O₁₂, obtained in two heating and cooling cycles, are shown forthe thick pellet in FIG. 2 a. There is no appreciable change in theconductivity between the two cycles. This means that the garnet-likestructure examined is thermally stable and that no phase transitionoccurs in the temperature range examined, viz. from room temperature to350° C. A similar Arrhenius behaviour was also observed for the thinpellet of Li₇La₃Zr₂O₁₂. In FIG. 2 b, the data for the thick pellet andthe thin pellet of Li₇La₃Zr₂O₁₂, which were in each case obtained in thefirst heating run, are compared. The activation energies obtained forthe volume and total conductivity of the thin pellet (0.32 eV at 18-50°C. and 0.30 eV at 18-300° C.) are slightly lower than those for thevolume and total conductivity of the thick pellet (0.34 eV at 18-70° C.and 0.31 eV at 18-300° C.). The conductivity obtained for the thinpellet is slightly higher than that of the thick pellet.

In addition to the impedance analysis, the ionic nature of theelectrical conductivity was also confirmed by EMF measurements in whichLi₇La₃Zr₂O₁₂ was used as solid electrolyte between elemental lithium andAl, LiAl. The sample was covered with an aluminium layer on the upperside and placed on lithium which had been melted in a glove box filledwith inert Ar gas. The aluminium was alloyed both by chemical reactionwith lithium and also by coulometric titration of lithium into thealuminium from the lithium electrode located opposite it. The resultingvoltage was in the region of the theoretical value. The difference couldbe attributed to the inhomogeneous temperature distribution andcorresponding phenomena due to irreversible processes.

FIG. 3 shows a comparison of the lithium ion conductivity ofLi₇La₃Zr₂O₁₂ with other known lithium ion conductors which are underconsideration for use in connection with batteries. The conductivity ishigher than that of Li-β-aluminium oxide, thin-layer Lipon(Li_(2.9)PO_(3.3)N_(0.46)), Li₉SiAlO₈, Lil+40 mol Al₂O₃, LiZr₂(PO₄)₃,Li_(3.5)Si_(0.5)P_(0.5)O₄, Li₅La₃Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂. The highlithium conductivity compared to other lithium-containing garnets andlow activation energy which are observed are possibly due to theincrease in the cubic lattice constants, the increase in the lithium ionconcentration, a reduced chemical interaction between the lithium ionsand other ions which form the lattice and partly to the improveddensification (92% of the theoretical density). At relatively lowtemperatures, the conductivity of the less stable polycrystalline Li₃N(6.6×10⁻⁴ S/cm at 27° C.) is comparable with that of Li₇La₃Zr₂O₁₂.However, at higher temperature, Li₇La₃Zr₂O₁₂ displays a higher totalconductivity.

The thermal stability of Li₇La₃Zr₂O₁₂, which is the fundamentaladvantage of the crystalline lithium ion conductor, was confirmed bythermogravimetric measurements (TGA) and differential thermal analysis(DTA). The TG-DTA data measured in an air atmosphere indicated nosignificant change in the mass and no discernible phase change bothduring heating and during cooling within the temperature range from 20to 900° C. It was found that zirconium-containing Li₇La₃Zr₂O₁₂ is stabletowards molten lithium and is also chemically stable to the action ofmoisture and air over the observation period of several weeks.

TABLE 1 Impedance data for Li₇La₃Zr₂O₁₂ (thick pellet: 1.02 cm thick and0.92 cm in diameter, and thin pellet: 0.18 cm thick and 0.98 cm indiameter), measured in air. Type of Temperature σ_(volume) σ_(total)pellets (° C.) (Scm⁻¹) (Scm⁻¹) R_(gb)/R_(b) + R_(gb) ^([a]) Thick 183.37 × 10⁻⁴ 1.90 × 10⁻⁴ 0.44 pellet 25 4.67 × 10⁻⁴ 2.44 × 10⁻⁴ 0.48 501.19 × 10⁻³ 6.15 × 10⁻⁴ 0.49 Thin 18 3.97 × 10⁻⁴ 2.32 × 10⁻⁴ 0.42 pellet25 5.11 × 10⁻⁴ 3.01 × 10⁻⁴ 0.41 50 1.45 × 10⁻³ 7.74 × 10⁻⁴ 0.47^([a])R_(gb) = grain boundary resistance, R_(b) = volume resistance

TABLE 2 Powder XRD pattern of Li₇La₃Zr₂O₁₂ having a garnet structure h kl d_(obs.) (Å) d_(cal.) (Å) I_(obs) 2 1 1 5.278 5.294 99 2 2 0 4.5744.584 17 3 2 1 3.460 3.465 55 4 0 0 3.239 3.242 42 4 2 0 2.897 2.899 1003 3 2 2.761 2.764 14 4 2 2 2.645 2.647 61 5 2 1 2.365 2.367 49 5 3 22.102 2.103 37 6 2 0 2.048 2.050 11 6 3 1 1.911 1.912 16 4 4 4 1.8701.871 12 6 4 0 1.798 1.798 21 6 3 3 1.764 1.764 20 6 4 2 1.732 1.732 937 3 2 1.646 1.646 15 8 0 0 1.620 1.621 14 6 5 3 1.549 1.550 13 7 5 21.468 1.468 9 8 4 0 1.450 1.449 11 8 4 2 1.415 1.414 17 9 2 1 1.3981.398 14 6 6 4 1.382 1.382 14 9 3 2 1.337 1.337 24 7 7 2 1.283 1.284 107 6 5 1.236 1.236 11 8 6 4 1.204 1.204 21 10 4 2 1.184 1.183 14 10 5 11.155 1.155 13 8 8 0 1.146 1.146 10 10 5 3 1.120 1.120 10

1.-20. (canceled)
 21. A process of producing an electrode whichcomprises coating a solid ion conductor having a garnet-like crystalstructure on an electrode or a protective layer before an electrode. 22.An electrode which has been coated with a solid ion conductor having agarnet-like crystal structure.
 23. A battery comprising one or moreelectrodes according to claim
 22. 24. A solid ion conductor which has agarnet-like crystal structure and has the stoichiometric compositionL_(7+x)A_(x)G_(3−x)Zr₂O₁₂, where L is in each case independently amonovalent cation, A is in each case independently a divalent cation, Gis in each case independently a trivalent cation, O≦x≦3 and O can bepartly or completely replaced by divalent or trivalent anion.
 25. Thesolid ion conductor according to claim 24, wherein O≦x≦1 and O can bepartly or completely replaced by divalent anion or N³⁻.
 26. The solidion conductor according to claim 24, wherein each L independently is Li,Na or K.
 27. The solid ion conductor according to claim 26, whereinL=Na.
 28. The solid ion conductor according to claim 24, wherein A is adivalent alkaline earth metal cation.
 29. The solid ion conductoraccording to claim 24, wherein A is Ca, Sr and/or Ba.
 30. The solid ionconductor according to claim 24, wherein the stoichiometric compositionis Li₇La₃Zr₂O₁₂.
 31. A process for preparing the solid ion conductoraccording to claim 24, which comprises reacting salts and/or oxides ofL, A, G and Zr with one another.
 32. The process according to claim 31,wherein the reaction is carried out by means of a precursor method. 33.The process according to claim 31, wherein the reaction is carried outby means of the Pecchini method, the glycine method and by means ofprecipitation reactions of dissolved salts of the components.
 34. Theprocess according to claim 31, wherein the reaction is carried out in asolid-phase reaction.
 35. The process according to claim 31, wherein Land A are used in the form of nitrates, carbonates or hydroxides and arereacted with G₂O₃ and ZrO₂.
 36. The process according to claim 31, whichcomprises the following steps: a) mixing of the starting materials andball-milling, b) heating of the mixture from a) at from 400 to 1000° C.in air for from two to ten hours, c) ball-milling, d) isostatic pressingof the mixture to produce a desired shape and e) sintering of theproduct from step d) covered with a powder of the same composition atfrom 700 to 1200° C. for from 10 to 50 hours.
 37. The process accordingto claim 31, which comprises the following steps: a) mixing of thestarting materials and ball-milling, in 2-propanol using zirconium oxidecontainers and balls, b) heating of the mixture from a) at from 400 to1000° C. in air for from two to ten hours, c) ball-milling, in2-propanol using zirconium oxide containers and balls, d) isostaticpressing of the mixture to produce a desired shape and e) sintering ofthe product from step d) covered with a powder of the same compositionat from 700 to 1200° C. for from 10 to 50 hours.
 38. The processaccording to claim 37, wherein the mixture is heated at 700° C. for sixhours in step b) and is sintered at 900° C. for 24 hours in step e). 39.An article which comprises the solid ion conductor as claimed in claim24, wherein the article is a battery, an accumulator, a supercap, a fuelcell, sensor, a thermoelectric converter or an electrochromic device.40. The electrode according to claim 22, wherein the garnet-like crystalstructure has the stoichiometric composition L_(7+x)A_(x)G_(3−x)Zr₂O₁₂,where L is in each case independently a monovalent cation, A is in eachcase independently a divalent cation, G is in each case independently atrivalent cation, O≦x≦3 and O can be partly or completely replaced bydivalent or trivalent anion.
 41. A battery comprising one or moreelectrodes according to claim 40.